SINGLE-CELL ANALYSIS OF THE CHLOROPLAST REDOX STATE REVEALS A LINK BETWEEN EARLY OXIDATION AND CELL FATE IN MARINE DIATOMS

Diatoms are photosynthetic microorganisms of great ecological and biogeochemical importance, contributing about 40% of marine primary production. They can form vast blooms that are frequently characterized by “boom and bust” dynamics. These dynamics include cycles of rapid proliferation of the population followed by a coordinated demise often accompanied by programmed cell death. However, the molecular basis and cellular mechanisms that underline the ecological success of diatoms are still underexplored.

Recent studies in our lab demonstrated how sub cellular oxidation patterns in response to environmental stress conditions may play an important role in cell fate determination in the diatom Phaeodactylum tricornutum. Here, we aim to further investigate the phenotypic variability within diatom populations. To this end, we combined flow cytometry and microfluidics fluorescence microscopy and measured organelle-specific oxidation dynamics at the single-cell level using the redox-sensitive sensor roGFP2. We revealed two distinct sub-populations in cells expressing chloroplastic roGFP2, exhibiting a bi-stable response in their redox state in response to oxidative stress. Cell death was subsequently induced in the oxidized sub-population, while the reduced sub-population resisted the oxidative stress. We further characterized an early phase in the response of cells to oxidative stress (~100 minutes) after which there was an irreversible induction of a cell death cascade.

We propose that intra-species phenotypic heterogeneity among individual diatom cells may provide an ecological strategy to cope with rapid changes in environmental stress in the marine ecosystem.